Don't extract extra tgz's until / is mounted r/w

This should mean the tardirs are now extracted and we can update those if so
desired as well as write to / if need be.
Add a '/flashrd.site' hook after onetime.tgz has been extracted

While it is possible to put everything staticly into onetime.tgz, some things
are easier to do in a script such as updating /etc/rc.conf.local.

An example of what a flashrd.site might contain:

ping -c 1 192.168.40.1 > /dev/null || dhclient em0
export PKG_PATH=ftp://ftp.openbsd.org/pub/OpenBSD/`uname -r`/packages/`machine`
pkg_add nrpe nagios-plugins check_hw_sensors
pkg_add nsh

echo sshd_flags=NO >>/etc/rc.conf.local
echo pf=NO >>/etc/rc.conf.local
echo inetd=NO >>/etc/rc.conf.local

echo "/usr/local/bin/nsh -i /flash/nshrc" >> /etc/rc.local
rm /usr/local/bin/save.sh
sed -e 's/etc/flash/' < /usr/local/share/save-rw.sh > /usr/local/bin/save.sh